Digital-to-Analog converters (DACs) are in widespread use today in electronic applications for consumer, medical, industrial, etc. Typically, DACs include circuitry for receiving a digital value and outputting an analog signal that is determined by the digital value. The digital output value is typically in the form of either a parallel word or a serial digital bit string. There are many types of digital-to-analog conversion schemes and, each of these conversion schemes has its advantages and disadvantages.
One type of DAC that has seen increasing use is the Sigma-Delta DAC (Sigma-Delta and Delta-Sigma will be used interchangeably herein). The Sigma-Delta DAC utilizes a Sigma-Delta modulator where digital values are input to the Sigma-Delta modulator and the output thereof is filtered to produce the analog signal and remove noise, e.g., an audio class-D power amplifier. A Sigma-Delta modulator in a DAC typically converts input digital values to a digital serial string of “ones” and “zeros” having an average amplitude over time proportional to the analog signal represented by the digital values. This digital serial string is filtered with an analog circuit low pass filter to produce the desired analog signal. Sigma-Delta modulation generally provides for high accuracy and wide dynamic range as compared to earlier delta modulation techniques. Sigma-Delta modulation is often referred to as an oversampled converter architecture and is typically immune from some of the earlier undesirable second order effects of delta modulation.
All Sigma-Delta modulators, working in a continuous mode, produce idle tones at their outputs if a certain periodic or DC input is provided. These idle tones are due to the quantization process and are inherent in the design of the Sigma-Delta modulator architecture.
These idle tones are unwanted and create undesired behavior at the output of the DAC, e.g., undesired high tones in an audio device. These tones limit the spurious free dynamic range (SFDR) and thus the signal-to-noise-and-distortion (SINAD) of the DAC device. These tones have an amplitude and frequency that is largely dependent on the input signal which prevents their removal with a simple filtering of the output signal.